When driving a vehicle, particularly in an off-road scenario, it is important for the vehicle's driver to be able to precisely and constantly control the vehicle's speed. This may be difficult to do in some operating circumstances. For example, in an off-road context, additional engine torque is required as the vehicle is often climbing a rock or other obstacle. However, immediately after the vehicle summits the obstacle, substantially less torque is required to maintain an approximately constant vehicle speed. In fact, an increase of brake pressure may be required to maintain a constant speed when descending the obstacle. This transition from increased torque to reduced torque and braking can happen very rapidly, making it difficult for the driver to maintain an approximately constant speed. Manual transmission vehicles typically combat this problem by employing low gears that enable the vehicle to climb the obstacle with little or no throttle application by the driver. Likewise, inherent engine braking in a low gear ratio manual transmission means that little to no conventional brake application is necessary to maintain the constant speed when descending the obstacle.
A typical automatic transmission, however, does not have as low a gear ratio as a typical manual transmission. Thus, both throttle and braking by the driver are necessary to maintain an approximately constant speed when driving over obstacles. Some prior art automatic transmissions have accomplished this type of control with the use of a driver operated dial that allows the driver to select from several preset speeds at which the vehicle may be instructed to travel. The vehicle's electronic control unit typically manipulates engine torque and braking to cause the vehicle to move at one of the preset speeds. However, in these designs, the driver is unable to control the speed of the vehicle using the throttle and brake pedal. Instead, the vehicle must travel at one of the preset speeds.
What is needed, therefore, is a method of operating a vehicle transmission to smoothly and accurately control vehicle speed during off-road driving. What is further needed is a method of manipulating the engine torque and brake pressure of a vehicle in response to driver throttle and brake commands to allow driver override if the driver wishes to increase or decrease the vehicle speed during operation.